Limitations of Marine Radar

1. Range Discrimination-

The RADAR can clearly distinguish two targets on the same bearing with a slight difference in ranges as two separate targets.

The factor which governs range discrimination is PL. Pulse length makes all targets appear larger in radial depth by an amount equal to half PL in meters. If the gap between the two targets is less than ½ PL in meters, then these targets would appear as one target on the PPI.

As per IMO Performance standards for RADAR, two small targets on the same bearing and separated by a distance of 40 meters on 1.5 miles range scale (placed at 50 to 100% of PPI), shall appear as two separate targets on the PPI.

2. Bearing Discrimination

A RADAR’s ability to distinguish two targets on the same range is slightly different in bearings as two separate targets.

The factor that governs bearing discrimination is HBW, which causes all targets to appear larger in Azimuth by equal to half HBW on either side, i.e. one full HBW. So if the angle at the Electronic Centre made by the inner corners of two targets is equal to or less than HBW their paints would merge on the PPI and they would appear as one big target.

Bearing discrimination is expressed in degrees and as per Performance Standards for Navigational RADAR (IMO), it should not exceed 2.5º.

3. Factors Affecting Minimum Range

The minimum detection range of the RADAR set depends on a. The Pulse Length b. The De-ionisation Delay c. The VBW And The Height Of The Scanner d. The Wavelength

Factors Affecting Maximum Range
a. Height Of Scanner
b. Power Of The Set
c. Wavelength
d. Pulse Repetition Frequency
e. Pulse Length
f. VBW & HBW
g. Receiver Sensitivity
h. Nature Of Target
i. Weather Effect
j. Anomalous Propagation
k. Sea and swell

5. Range Accuracy

As per Performance Standards for Navigational RADAR (IMO), the error in the range of an object obtained using range rings or the VRM, should not exceed 1% of the maximum range of the scale in use, or 30 m whichever is greater.

The range accuracy of RADAR depends on:
a. Correct synchronization between the transmission of the pulse and the commencement of the trace. b. Uniformity And Rectilinearity of the time base.
c. The Scale Of Size Of The Tracing Spot
d. Height Of Scanner

6. Bearing Accuracy

As per Performance Standards for Navigational RADAR (IMO), the RADAR bearing of an object whose echo appears on the edge of the display should be capable of being measured with an accuracy equal to, or better than +/- 1º.
Factors that govern bearing accuracy are:
a. Correct alignment between the heading marker and the scanner.
b. Correct alignment bet. The heading marker and the bearing scale.
c. Gyro error, if any when the display is Gyro-stabilised.
d. Rectilinearity of the trace.
e. Beamwidth distortion.
f. Scale the size of the spot.

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